This disclosure pertains to microlithography, which is a key technique used in the manufacture of microelectronic devices such as semiconductor integrated circuits, displays, and the like. More specifically, the disclosure pertains to reticles for use in microlithography performed using a charged particle beam such as an electron beam or ion beam, wherein the reticle defines a pattern to be transferred lithographically to a suitable substrate. Even more specifically, the disclosure pertains to determining the pattern to be defined on the reticle.
As the degree of integration of active circuit elements in microelectronic devices has continued to increase, with corresponding decreases in the size of individual active circuit elements in such devices, the resolution limitations of conventional optical microlithography increasingly have become apparent. Consequently, substantial effort is being expended to develop a practical xe2x80x9cnext generationxe2x80x9d microlithography (NGL) technology. One promising candidate NGL technology is microlithography performed using a charged particle beam, which offers prospects of better resolution than optical microlithography for reasons similar to reasons for which electron microscopy yields better image resolution than optical microscopy. Charged-particle-beam (CPB) microlithography can be performed using an electron beam or ion beam. Most effort is being expended to develop a practical electron-beam microlithography apparatus.
With current CPB microlithography apparatus, it is not possible to transfer-expose an entire pattern or even a large portion thereof in a single exposure xe2x80x9cshotxe2x80x9d due to various factors such as the aberration and distortion exhibited by conventional CPB optical systems. For this reason, transfer-exposure using a xe2x80x9cdividedxe2x80x9d reticle has been developed. In a divided reticle, the pattern (corresponding in area to one xe2x80x9cchipxe2x80x9d or xe2x80x9cdiexe2x80x9d on the lithographic substrate) as defined on the reticle is divided, or xe2x80x9csegmented,xe2x80x9d into a large number of exposure units, usually termed xe2x80x9csubfields,xe2x80x9d that define respective portions of the pattern. Exposure of the pattern from the reticle occurs subfield-by-subfield, wherein the respective images of the subfields are transferred to respective locations on the substrate such that the individual subfield images are xe2x80x9cstitched togetherxe2x80x9d in a contiguous manner to form the desired chip or die on the substrate. Typically, multiple chips are formed on a single substrate. So as to be imprintable with die patterns, the upstream-facing surface of the substrate is coated with a thin film of a substance termed a xe2x80x9cresist.xe2x80x9d
A typical manner of dividing the pattern into subfields is shown in FIG. 8. First, as noted above, multiple chips are transfer-exposed onto a xe2x80x9ctransfer bodyxe2x80x9d or lithographic substrate (usually a semiconductor xe2x80x9cwafer,xe2x80x9d which is the term used herein). The chip pattern, as transferred, is divided into one or more xe2x80x9cstripes,xe2x80x9d and each stripe is subdivided into multiple subfields. The respective subfields in each stripe are arranged rectilinearly in multiple rows, each containing multiple respective subfields. The pattern on the reticle, and thus the reticle itself, similarly is divided into stripes and subfields.
Transfer-exposure performed using a CPB microlithography apparatus and a divided reticle typically is performed in a manner as shown in FIG. 9. First, the reticle and wafer are mounted on respective stages that provide support and controlled movements of the reticle and wafer, respectively, as required for exposure. During exposure, the respective stages position the reticle and wafer such that the optical axis of the CPB optical system intersects the respective centerlines of the selected stripe on the reticle and wafer. Exposure of a stripe is achieved by appropriate lateral deflections of the beam (performed by the CPB optical system), accompanied by respective continuous motions of the stages at respective constant velocities along the respective stripes, to expose the subfields in the selected stripe subfield-by-subfield and row-by-row.
The respective stage-movement velocities roughly correspond to the xe2x80x9cdemagnificationxe2x80x9d (reduction) ratio of the portion of the CPB optical system used to form the images on the wafer. For example, with a demagnification ratio of 1/4, each subfield image formed on the wafer is 1/4 the size of the respective subfield on the reticle; hence, during exposure the wafer stage moves at about 1/4 the velocity of the reticle stage.
For exposure, the CPB optical system includes an xe2x80x9cillumination-optical systemxe2x80x9d for illuminating the subfields on the reticle and a xe2x80x9cprojection-optical systemxe2x80x9d for carrying respective aerial images of the illuminated subfields to the wafer and for resolving the images on the surface of the wafer. The charged particle beam propagating through the illumination-optical system is termed the xe2x80x9cillumination beam,xe2x80x9d and the charged particle beam propagating through the projection-optical system is termed the xe2x80x9cpatterned beamxe2x80x9d or xe2x80x9cimaging beam.xe2x80x9d
Thus, during exposure of a stripe, the illumination beam is deflected laterally in a direction approximately perpendicular to the reticle-stage-movement direction to expose each row subfield-by-subfield. As exposure of a particular row ends, respective stage movements bring the next row into position for exposure, with a corresponding reverse in the deflection direction of the beam to expose the constituent subfields of the new row, and so on to the end of the stripe. Hence, exposure of the stripe progresses in a raster manner, which minimizes time lost between exposures of adjacent rows and thereby increases throughput. As exposure of a particular stripe ends, respective stage movements bring the next stripe into position for exposure.
The reticle used in the exposure method described above differs substantially in structure from a reticle used for optical microlithography. Whereas a reticle for optical lithography can be exposed in a single xe2x80x9cshotxe2x80x9d and is self-supporting, the reticle for CPB microlithography is structured to define individual subfields (each defining a respective portion of the pattern) and intervening structural members termed xe2x80x9cstruts.xe2x80x9d The struts extend across the reticle in a lattice manner and separate the subfields one from another. Contiguous with the struts are frame members extending around the circumference of the reticle. The struts and frame provide structural strength and rigidity for the reticle. Each subfield on the reticle includes a respective membrane portion that includes a respective patterned portion and a respective skirt. The patterned portion defines the respective portion of the pattern defined by the reticle. The skirt surrounds the patterned portion. The patterned portion is transmissive to the illumination beam such that, as the illumination beam passes through the patterned portion, the beam acquires an aerial image of the respective pattern elements defined in the patterned portion. The outer edges of the illumination beam fall within the skirt as each subfield is illuminated. The skirt and the struts surrounding the skirt effectively isolate each subfield for individual exposure without crosstalk between adjacent subfields during exposure.
CPB microlithography is subject to a phenomenon known as the space-charge effect (also termed a xe2x80x9cCoulomb effectxe2x80x9d) caused by mutual electrostatic repulsion of charged particles in the beam. The mutual repulsion causes widening of the beam, with an accompanying drop in pattern-transfer resolution. To reduce the space-charge effect, the beam-acceleration voltage may be increased to increase the velocity of particles in the beam and correspondingly reduce the particlexe2x80x94particle interaction time during propagation from the reticle to the wafer. Hence, increasing the beam-acceleration voltage conventionally is a favored means for increasing pattern-transfer resolution.
However, increasing the beam-acceleration voltage causes certain problems, notably undesired changes in the profiles of pattern elements as transfer-exposed onto the wafer, especially in peripheral regions of the chip. For example, a higher beam-acceleration voltage can cause undesired increases in pattern linewidth, relative to design-specified values, especially in peripheral regions of the chip, compared to similar exposures using a lower beam-acceleration voltage. As a result, pattern resolution of the overall chip is degraded.
In view of the shortcomings of conventional methods as summarized above, the present invention provides, inter alia, improved methods for configuring a pattern on a reticle. The methods result in pattern portions destined to be located on or near peripheral regions of chips being transferred with greater fidelity to design-mandated values, even when transfer-exposed using a charged-particle-beam (CPB) microlithography (xe2x80x9cexposurexe2x80x9d) apparatus utilizing a high beam-acceleration voltage.
According to a first aspect of the invention, methods are provided for configuring a reticle pattern to be defined on a reticle used for charged-particle-beam microlithography. An embodiment of such a method comprises the step of identifying an element of the pattern destined for transfer-exposure to a region of a chip formed on a lithographic substrate, wherein the pattern element has an initial configuration. In another step the pattern element as defined on the reticle is reconfigured such that the profile at least partially offsets proximity effects that otherwise would be imparted to the pattern element, if the element were to be transfer-exposed in its initial configuration to the chip, by proximal elements of the pattern transfer-exposed to the same chip and by proximal elements located in an adjacent chip or chips on the substrate. The reconfigured pattern element desirably is defined in one or more subfields of the reticle. xe2x80x9cProximal elementsxe2x80x9d are elements located within a xe2x80x9cproximal rangexe2x80x9d, as defined herein later below, of a subject pattern element.
Another embodiment includes the element-identifying step summarized above. A determination is made of a net proximity effect that otherwise would be imparted to the pattern element, if the element were to be transfer-exposed in its initial configuration to the chip, by proximal elements of the pattern transfer-exposed to the same chip and by proximal elements located in one or more adjacent chips on the substrate. The element as defined on the reticle is reconfigured so as to offset the net proximity effect at least partially.
Yet another embodiment also includes the element-identifying step summarized above. A determination is made of a net proximity effect that otherwise would be imparted to the pattern element, if the element were to be transfer-exposed in its initial configuration to the chip, by at least one proximal element of the pattern transfer-exposed to the same chip and by at least one proximal element located in one or more adjacent chips on the substrate. A calculation is made of a profile change to be made to the pattern element, as defined on the reticle, that would offset the net proximity effect at least partially and cause the pattern element, when transfer-exposed to the substrate, to be substantially similar to a corresponding design-mandated profile for the element. The profile of the pattern element is changed according to the calculated profile change, and the pattern element is defined on the reticle according to the changed profile. This method can include the steps of determining a manner in which the pattern is to be divided, on the reticle, into subfields, and defining the pattern element in at least one subfield.
According to another aspect of the invention, methods are provided for manufacturing a divided reticle for use in charged-particle-beam microlithography. An embodiment of such a method comprises the step of dividing a pattern, to be defined on the reticle, into subfields each including a respective portion of the pattern. An identification is made of a pattern element destined for transfer-exposure to a region of a chip formed on a lithographic substrate, wherein the pattern element has an initial configuration. The pattern element is reconfigured so as to have a profile, as defined on the reticle, that at least partially offsets a net proximity effect that otherwise would be imparted to the pattern element, if the element were to be transfer-exposed in its initial configuration to the chip, by at least one proximal element of the pattern transfer-exposed to the same chip and by at least one proximal element located in one or more adjacent chips on the substrate. The reconfigured pattern element is defined in at least one subfield, and the reticle is fabricated to include the reconfigured pattern element.
In another embodiment of this method a pattern, to be defined on the reticle, is divided into subfields each including a respective portion of the pattern. A pattern element destined for transfer-exposure to a region of a chip formed on a lithographic substrate is identified. A determination is made of the net proximity effect that otherwise would be imparted to the pattern element, if the element were to be transfer-exposed in its initial configuration to the chip, by at least one proximal element of the pattern transfer-exposed to the same chip and from proximal elements located in adjacent chips on the substrate. The pattern element is reconfigured to have a profile that at least partially offsets the net proximity effect. The reconfigured pattern element is defined in at least one subfield, and the reticle is fabricated to include the reconfigured pattern element.
Yet another embodiment of this method includes the pattern-dividing step, the pattern-element-identification step, and the net-proximity-effect-determination step summarized above. A calculation is made of the reconfigured profile of the pattern element, as defined by the reticle, that would offset the net proximity effect at least partially and cause the pattern element, when transfer-exposed to the substrate, to be substantially similar to a corresponding design-mandated profile. The pattern element is reconfigured according to the calculation, the reconfigured pattern element is defined in at least one subfield, and the reticle is fabricated to include the reconfigured pattern element.
Another aspect of the invention is directed to divided reticles manufactured by any of the method summarized above.
According to another aspect of the invention, methods are provided for performing a microlithographic exposure using a charged particle beam. In an embodiment of such a method, a divided reticle is provided as summarized above, wherein the reticle defines a pattern divided among multiple subfields. A charged-particle illumination beam is directed subfield-by-subfield through the reticle, to produce a corresponding patterned beam. The patterned beam is directed to a resist-coated lithographic substrate so as to imprint the pattern in multiple chips on the substrate. The step of directing the patterned beam can comprise the step of imprinting the pattern in centrally located chips and in peripherally located chips on the substrate. In such an instance, the method can further comprise the step of reducing variations in the imprinted profile of the pattern element in the peripherally located chips versus in the centrally located chips on the substrate by transfer-exposing portions, of peripheral chips that extend partially off the substrate, of such peripheral chips still remaining on the substrate. With respect to the peripheral chips extending partially off the substrate, transfer-exposure can be made of at least one respective subfield of the portions of such peripheral chips still remaining on the substrate.
According to yet another aspect of the invention, methods are provided for manufacturing microelectronic devices, wherein the methods include a microlithographic-exposure method as summarized above.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.